Currently, electrically conducting polymers are subjected to in-depth research worldwide. These polymers offer the possibility of replacing metallic conductors and semiconductive materials in a variety of applications including batteries, transducers, switches, photocells, circuit boards, heating elements, antistatic protection (ESD) and electromagnetic protection (EMI). Conducting polymers possess, i.a., the following advantages over metals: light weight, advantageous mechanical properties, good corrosion resistance and lower cost of synthesis and fabrication.
Conducting plastics can be coarsely divided into two different cathegories: filled conducting plastics in which a thermosetting or thermoplastic resin is made conductive by the addition of a conductive filler such as, e.g., carbon black or lampblack, carbon fiber, metal powder, etc., and intrinsically conducting plastics based on polymers made conductive by an oxidation or reduction (doping) process.
The electrical conductivity of filled conducting plastics is dependent on the mutual contacts formed between the conductive filler particles. Typically, approx. 10-50% wt. of a well dispersed filler material is required to achieve composites of high conductivity. Such conducting composite materials have, however, several drawbacks: The mechanical and some chemical properties of such composites are decisively degraded with the increase in the filler content and decrease in the polymer proportion; their conductivity becomes difficult to control particularly in the semiconductive range; and a stable and homogeneous dispersing of their filler into the matrix polymer becomes difficult.
Intrinsically conducting plastics can be produced from organic polymers having long chains containing conjugated carbon-carbon double bonds or double bonds and heteroatoms. Stable .pi.-electron systems of double bonds and heteroatoms can be perturbed by adding to the polymer matrix certain doping agents which can be either of electron donor or acceptor type. Thus, the backbone chain of the polymer can be modified to contain electron holes and/or excess electrons that provide charge carriers for electric current along the conjugated chain.
The benefits of intrinsically conducting plastics include easy modification of their conductivity as a function of doping conditions, which is particularly accentuated in conjunction with low conductivities. Attainment of low conductivities with filled conducting plastics is difficult. Exemplifying kinds of intrinsically conducting polymers are polyacetylene, poly-p-phenylene, polypyrrole, polythiophene and polyaniline. The use of most intrinsically conducting plastics in the promising applications mentioned above is, however, limited by the inferior processability and stability properties of those polymers.
Generally, it can be expected that if it were possible to achieve a melt-processable compound material that combines the electrical properties of an intrinsically conducting polymer (for electrical conductivity) with the mechanical properties of a thermoplastic polymer, a conducting plastic material would result with superior characteristics over filled conducting plastic compound materials.
A technically and commercially promising intrinsically conducting polymer is polyaniline and its derivatives. An aniline polymer is based on an aniline unit in which the nitrogen atom is bonded to the para-carbon in the benzene ring of the next unit. Unsubstituted polyaniline can occur in several different forms such as leucoemeraldine, protoemeraldine, emeraldine, nigraniline and toluprotoemeraldine.
The so-called emeraldine base form of polyaniline is generally depicted with the molecular formula ##STR1## wherein x is approximately 0.5.
In similarity with almost all intrinsically conducting polymers, however, also polyaniline is plagued by poor processing properties which makes it difficult to fabricate this polymer into parts, films, fibers, etc. of desired form.
Polymers are generally processed along two major lines: melt processing and solution processing. Problems encountered in the melt processing of conductive polymers arise from the fact that the thermal decomposition of the polymer starts prior to its melting or plasticization. A problem in the solution processing of conductive polymers has been their low solubility in conventional industrially used solvents. Another problem arises from the fact that, even though solution processing is applicable to processing of films and fibers from which the solvent can be evaporated, fabrication of practical parts with a desired shape is very different through solution processing.
Patent publication EP 432,929 discloses a few alternatives aimed at solving the above processability problems. U.S. Pat. Nos. 3,963,498 and 4,025,463 describe polyaniline oligomers which have less than 8 aniline units and are soluble in certain organic solvents. These oligomers lack, however, the good mechanical and chemical properties characteristic of polymers.
The closest approaches to industrially useful methods suited to practical applications are described in the following patent publications: U.S. Pat. No. 5,006,278, WO 8,901,694, WO 9,013,601, WO 9,010,297, WO 9,001,775 and U.S. Pat. No. 5,002,700. Of these, U.S. Pat. No. 5,006,278 discloses a conducting material achieved by mixing a solvent, a doping agent and polyaniline, whereafter the solvent is removed from the mixture. The patent publication WO 8,901,694 discloses a polyaniline polymer suited to solution processing; the conductivity of this polymer is achieved by doping with sulfonic acid. Such a polyaniline according to the patent publication is applicable to processing conducting polymers with, e.g., the following matrix plastics: PE (polyethene), PP (polypropene), PA (polyamide). Patent publication WO 9,013,601 describes a method for producing a conducting polymer blend by first preparing a mixed blend of the polyaniline and a doping agent in a suitable solvent, then blending this mixture with a polyamide, after which the solvent is evaporated. The doping agent is an aromatic multi-sulfonic acid. According to this publication the doping is typically performed at approx. +20.degree.-+ 25.degree. C. The doping takes place in a heterogenic reaction, after which the blend is dissolved in a suitable solvent. The processing is carried out with some solvent still present (page 15, line 23), which acts as a plasticizer.
Furthermore, the use of dodecylbenzenesulfonic acid as the doping agent for polyaniline is known from patent publications WO 9,010,297 and U.S. Pat. No. 5,002,700. On the other hand, patent publication WO 9,001,775 suggests the use of a multi-sulfonic acid as the doping agent for polyaniline featuring an improved temperature stability over other kinds of sulfonic acids. The examples of the abovementioned WO patent publication were carried out using a doping temperature of +20.degree.-+25.degree. C. at atmospheric pressure and the doping was carried out as a suspension of the polyaniline and the sulfonic acid in an aqueous solution of formic acid.
Publications related to the state of the art suggest rather widely that such processing methods are applicable to a variety of derivatives of polyaniline, and moreover, to be suited to both solution and melt processing. The exemplifications presented in the publications indicate, however, that practical processing conditions for polyaniline preprotonated with a suitable doping agent have been found only in some solution processing arrangements. In all examples of the abovementioned publications it has been necessary to dissolve the suspension of the polyaniline and the doping agent into a suitable solvent in order to achieve the homogenization of the blend. Evidently, this has been necessary in order to make the polymer blend sufficiently homogeneous and conducting. In any case, it is obvious that no processing methods suited to melt processing have been presented in prior art in the form of verified examples.
Melt processing also has the drawback that the doped polyaniline or the blend of polyaniline and a doping agent can generally be processed only once. This limitation makes it hard to produce a homogeneous melt-processable material in the form of, e.g., a granulate. Therefore, in prior-art melt processing tests the components have been mechanically blended just prior to their melt-processing into the final shape. Samples obtained from the tests have been characterized by varying conductivity and inhomogeneous quality.
As is evident from the state of the art, a mere blending of a solid polyaniline of the emeraldine base form and a doping agent (e.g. dodecylbenzenesulfonic acid, DBSA) fails to achieve a homogeneous doped polyaniline. The blending gives a so-called preprotonated polyaniline-dopant blend. Such a liquid blend or dispersed suspension is in an undefined form which is messy, highly corrosive and difficult to handle. Hence, in the examples of prior-art publications, a sufficient mixing for doping the polyaniline has been implemented by dissolving and/or dispersing both components of the blend in a mutually compatible solvent.
The abovedescribed problem associated with sufficient doping is also reflected in the preparation of a compound material of doped polyaniline and a thermoplastic polymer. Thus, the initial tests performed in conjunction with the present invention have indicated that a simple blending of a thermoplastic polymer with polyaniline and DBSA acting as the doping agent fails to give a homogeneous product which can be melt-processed into an electrically conducting part of desired shape. Test pieces made according to this method have visible defects such as various flow marks, blisters and cracks in addition to poor chemical properties. The problem arises from the poor incompatibility of the polyaniline and/or the doping agent or a doped polyaniline with a thermoplastic polymer during melt-processing conditions.